I run my analog telco over cat5, but that's in-house and definitely not 3km. That sounds really far for current loop stuff.

I was doing that too. I asked this same question a few years ago and
the answer was 100-200 meters. This is just a quick rule of thumb, but
it seems about right. 3km, I doubt that would work, but it depends, as
someone said, totally depending on ohm's law :)

What I think about this is, the length of the copper cable between the central office and home is usually several km, but definitely helped by the central office circuitry (current source instead of voltage source, that guarantees a minimum ringing voltage on the far end). What I don't know is, a FXS port behaves the same as a central office, electrically speaking? If that is so, you could extend your 3km of cable without problems, but I think you can have some noise problems depending on what places the cable has to go through.